It was for 80 users, so not a viable site for us to write a full-state migration.
Always wanting to put useful information up here we thought we'd sketch how to do small migrations like this on your own. Since last I looked at iCalServer 1.0.6, Apple Calendar Admin did not make migration an easy thing.
The Quick and Dirty Way
The Right Way
The right way is where we read the iCalendar calendar data store and insert into Office 365 of Exchange directly. I.e., the right way is server-to-server rather than script-to-client. This also allows users to be remapped, and then inserted into Office 365 / Exchange while maintaining full-state calendar information
This amount of engineering is impossible to justify for small sites.
If however, you have a few thousand users you need to migrate feel free to contact us.
(Later addition: We did this, as the video shows)